1. Field of Invention
The present invention relates to optically active polyesteramides, photoreactive chiral agents causing a change of liquid crystal structure, and liquid crystal compositions, optical films, liquid crystal color filters and recording media containing the photoreactive chiral agents, as well as a method for changing or fixing the helical structure of liquid crystals.
2. Description of the Related Art
In recent years, liquid crystal materials have attracted a great deal of attention. For example, cholesteric liquid crystal compounds, which have a helical structure and display colorful selective reflection colors due to the twisting power (twisting angle) of the helix, are excellent in selective reflectance and in color purity of selective reflected ray, and have widely been used in optical films, liquid crystal color filters and recording media.
The following is a specific example of color filters.
For example, color filters used in color liquid crystal displays are comprised of respective pixels of red (R), green (G) and blue (B) and black matrix formed in gaps between the pixels for improving the display contrast. Such color filters conventionally employed include mainly those made by dispersing a pigment or pigments into to resins or dyeing resins with dyestuffs. In producing such color filters, it was general to apply a color resin solution on a glass substrate by spin coating to form a colored resist layer followed by patterning of the layer by a photolithography technique to form color filter pixels, or to print colored pixels directly on a substrate.
In the printing process, however, there is a disadvantage that it is difficult to form a very fine image pattern because of a low resolution of the pixels. In the spin coat process, there are disadvantages of a large loss of materials and when applied to a substrate having a large area coating non-uniformities are large. Furthermore, in an electrodeposition process, it is possible to obtain a relatively high resolving color filter with few non-uniformities in the color layer, but there is a disadvantage that the manufacturing process is complicated and managing the solutions used in this process is difficult.
Therefore, provision a process for conveniently producing color filters of high quality in high efficiency with little loss of raw materials has been desired.
On the other hand, as for the performance of the color filters, high transmittance and high color purity have been required. In recent years, in order to satisfy such requirements, it has been attempted to optimize the kind of dyestuffs or staining resins in a process of using dyestuffs or to use a finely dispersed pigment in a process of using pigments. In a current liquid crystal display (LCD) panel, however, a demand for the transmittance and color purity of color filters is very high, particularly in a reflex LCD color filter it is difficult to achieve both paper white display/contrast and color reproducibility. On the other hand, since all color filters prepared by depositing dyestuffs in resins or dispersing a pigment in resins according to the conventional process are of light absorption type, an improvement of color purity by means of further enhancement of transmittance has substantially reached the upper limits.
In such a situation, a color filter of polarization type comprising cholesteric liquid crystal as major component has been known. This color filter of polarization type, since it displays an image by reflecting a certain quantity of light and transmitting the rest, has a high utilization efficiency for light and much higher performance with respect to transmittance and color purity than color filters of the light absorption type. On the other hand, in producing such a filter, it is usual to employ a spin coating process to form a film on a substrate since the resulting film has uniform thickness, however, the spin coating method is disadvantageous in cost because of a great loss of materials.
In order to solve the above-mentioned problems, as a means, which ensure uniformity of color purity on the color filter film and can realize reduction of the production steps, methods using chiral compounds of photoreactive type are effective. This process is based on the principle that when a liquid crystal composition containing a chiral compound of photoreactive type is irradiated with light at the reactive wavelength of the chiral compound in a pattern shape, the reaction of the chiral compound proceeds in accordance with the intensity of irradiation energy to change the helical pitch (helical twisting angle), resulting in formation of selective reflection colors for every pixel only by pattern exposure different in the quantity of light. Thus, there is an advantage that the patterning in formation of a color filter may be completed by one mask exposure using a mask having different quantities of transmitted light.
Therefore, after patterning by image wise irradiating light, the patterned cholesteric liquid crystal compound can be fixed to form a film acting as a color filter. This technique can be applied to optical films or record of images.
Particularly, when a color filter is made by single mask exposure, being able to form the primaries of B (blue), G (green) and R (red) with high color purity in a single exposure is desired. When the rate of twisting change in liquid crystals is small, however, sufficient color purity cannot be attained. Therefore, in order to make the primaries display by single exposure with high color purity, it is necessary to use a highly twisted chiral compound (chiral agent) which can change significantly the twisting power of the helical structure in liquid crystals. In other words, the use of a highly twisted chiral compound allows expansion of the width of the hue, which is selectively reflected depending on the change of light quantity.
As for such chiral agents, the present applicant has previously filed patent applications involving low molecular photoreactive chiral agents having an isosorbide skeletal structure (Japanese Patent Application No. 2001-5740) and high molecular photoreactive chiral agents (Japanese Patent Application No. 2001-144532). Moreover, WO 00/34808 discloses low molecular photoreactive chiral agents having a benzylidene menthone skeletal structure.
On the other hand, when the cholesteric liquid crystal compound is fixed after patterning by imagewise irradiation of light, an image with good resolution can not be obtained if the pattern is not maintained during the interval from after patterning to fixing.
The light source used in mask exposure is usually an ultrahigh pressure mercury lamp having an emission line at 365 nm. Accordingly, it is desired to conduct the mask exposure with a chiral agent having the high molar absorption coefficient in this range of wavelength in order to enhance the reaction rate of the chiral compound at high sensitivity.
The above-mentioned chiral agents, however, cannot obtain an image having excellent sensitivity. Alternatively, there are problems that a molar absorption coefficient thereof is small at 365 nm decreasing the sensitivity during mask exposure and that the cis-isomer after photoisomerization is poor in thermal stability.